Gas removal with a carbon-water slurry



United States Patent GAS REMOVAL WITH A CARBON-WATER SLURRY FrankMaslan, Newton Highlands, Mass., assignor to Escambia Chemical.Corporation, Pace, Fla., a corporation of Delaware No Drawing. Originalapplication May 11, 1956, Serial No. 584,186. Divided and thisapplication June 11, 1957, Serial No. 664,914

4 Claims. (Cl. 183-1141) This invention relates to the production ofchemicals and in particular to the separation or recovery of chemicalsfrom gaseous mixtures. This application is a division of copendingapplication SerialNo. 584,186,.filed May 11, 195 6.

A principal object of the present invention, is to provide an improved,adsorption process for the separation of chemicals from gaseousmixtures.

Another object of the invention is to provide; a liquid adsorptionsystem which has many additional advantages over the currently employedadsorption systems.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the process involving the severalsteps and the relation, and the order of. one or more of such steps withrespect to each of the others which are exemplified in the followingdetailed disclosure, and the scope of the application of which will beindicated in the claims.

Fora fuller, understanding of the nature and objects of the invention,reference should be had to. the following detailed description.

Adsorption is usually carried out by passing a mixture of gases over anadsorbent such as charcoal. The ad sorbent may either be a stationarybed or a moving one. The adsorbent will adsorb varying amounts of thegases present in the mixture depending on their characteristics.Basically adsorbents such as charcoal will adsorb highmolecular-weight,low-volatility compounds in preference to low-molecular-weight,high-volatility ones.

Adsorption, when used as a gaseous separation process, has some verydefinite advantages. First, it is possible to get a very high recoveryefficiency. For. example, better than 95 percent of the propanecontained in a methane stream can be recovered without much difiiculty.Concurrently, only 5 to percent of the methane would be adsorbed.Second, the adsorption. process is highly selective. This is illustratedby comparison of the percentages of propane and methane, given in theprevious example. This high degree of selectivity enables one to make asharp separation. Third, comparatively low pressures are requiredin theadsorption process. These pressures are usually lower than forcomparable solvent absorption processes. An adsorbent such as activatedcharcoal is capable of adsorbing as. much. as percent of its weightof. amaterial such as. propane at pressures of 50 p. s. i. g. and less.Fourth, the adsorption process is usually carried out at roomtemperature. There is little need for refrigeration other-than thatobtained with cooling water. On the other hand, it is very common forrefrigeration to be required in absorption processes in order to obtaina high enough solvent capacity.

Most of the disadvantages of adsorption separation processes lie in thehandling requirements for the solid adsorbents. In the fixed-bedadsorber, over-all efficiency is low because all the adsorbent cannot beutilized and because the process is inherently a batch operation. Themoving-bed type adsorption process requires large-scale,

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expensive equipment and has a fairly high attrition loss of theadsorbent. This attrition problem is probably the chief drawback tousing any kind of moving adsorbent bed. In this process, particles aboutA inch diameter are usually used. As they move down the tower, theygrind on each other, creating a certain amount of fines which have to beseparated and discarded on recycle. Since adsorbents are quiteexpensive, these fines represent a significant percent of the operatingcost. The fluidizedbed type adsorption has an even worse attritionproblem. In this type of operation, the adsorbent has a usual mesh sizeof to 200 and is fluidized with the gaseous mixture so that it flowscountercurrent to the gas through a contacting device. As it does so,the adsorbent grinds on itself, producing a very high percent ofattrition. For many gaseous separation processes, it is desirable tooperate at elevated pressures. On the other hand, it is dithcult tooperate solid-gas contacting units at elevated pressures. This isprimarily a problem of maintaining gastight seals in solids-handlingequipment.

The adsorption processes, whether stationary bed orcountercurrent-moving bed, are inherently adiabatic operations. Due tothe head of adsorption, fairly high temperatures occur where theadsorption is taking place. Since the amount of gas adsorbed is directlyrelated to the temperature, any rise in temperature is undesirable. Onthe other hand, due to the poor heat transfer characteristics andexpensive equipment, cooling by such means as cooling water in tubes isnot usually resorted to in this equipment.

The present invention is primarily directed to a process for separating,recovering or purifying gaseous mixtures containing a. gas or gaseswhich can be adsorbed on an adsorbent by contacting the gaseous mixturewith an adsorbent suspended in a liquid, the adsorbent and liquid beingmutually incompatible. In one preferred embodiment of the invention, theliquid is water and the adsorbent suspended therein is charcoal,preferably activated charcoal. In another embodiment of the invention, aliquid-adsorbent slurry can be used for the separation of gases solublein the liquid from gases which can be adsorbed on an adsorbent.

The present adsorption separation process has all the advantages of theadsorbent systems described above and yet eliminates most of thedisadvantages found therein. Basically the present invention provides animproved adsorption medium which consists of an adsorbent suspended in aliquid so as to form a slurry. For example, finesize activated charcoalcan be suspended in water. Hydrodynamically this decreases almostcompletely the attrition of the particles on each other. This is due tothe fact that the liquid, such as Water, has a viscosity and surfacetension so much higher than air that each particle of adsorbent isshielded from the other particles by a layer of liquid. This markeddecrease in attrition eliminates the main economic reason for not usingfluid-bed adsorption processes. Depending on the system of adsorbent andliquid, a wetting agent may be used in order to get better contactbetween the liquid and the adsorbent. This will depend upon the specificsystem employed.

The present adsorption process is operable if the gaseous mixture to betreated can diffuse through the liquid and if the liquid and adsorbentare mutually incompatible. This is true of charcoal and water. Charcoalwill preferentially adsorb organic materials rather than water. On theother hand, such materials as activated alumina and silica gel arehydrophilic. In separating organic gases, the most useful system isactivated carbon or charcoal suspended in water.

Not only is the attrition of the adsorbent decreased by using the liquidbut a slurry is much easier to handle than a gas-solid mixture. may bepumped. Since the slurry is like a liquid, it can be handled easily atelevated pressures. Equipment and seals ordinarily used for gas-liquidsystems can easily be used for the gas-slurry systems discussed here. Ina fluidized bed system, large quantities of gas are required for movingthe fluidized adsorbent. This requirement is entirely eliminated byusing the slurry. The liquid (water) will act as a coolant for the heatof adsorption. This means that the temperature of adsorption can becontrolled to desirable lower temperatures. T his can mean a largedifierence in the adsorption capacity of a material such as charcoal.

The rich adsorbent in the slurry may be regenerated by one of severalmeans or combinations thereof. The ambient pressure may be decreased.This causes the adsorbate to come off of the adsorbent. The slurry maybe boiled. This is a very convenient way of retaining the slurry and, atthe same time, raising the temperature of the system so as to cause theadsorbed gas to come oil. The rich adsorbent can be separated from theliquid and then heated to as high a temperature as is required forregeneration. The separated adsorbent may be washed with a solvent whichremoves the adsorbate, or the adsorbate may be displaced with anothermaterial which is more preferentially adsorbed. A combination of heatingand decreasing of pressure is the most preferable regenerationtechnique.

The present adsorption process may be used, for example, in theextraction of ethane and propane from natural gas or refinery gas. Thiscan be easily accomplished by contacting natural gas, for example, witha slurry of water and activated charcoal at room temperature and at apressure of about 500 p. s. i. g. The ethane and propane are adsorbed bythe suspended activated charcoal and recovered therefrom by, forexample, decreasing the pressure to about 250 p. s. i. g. and heating soas to cause the adsorbates ethane and propane to come off of thecharcoal adsorbent.

The present adsorption process may also be used in the recovery orpurification of acetylene from Sachsse or Wultf type acetylene streams.Acetylene streams containing less than about 15 percent acetylene arenow being commercially produced directly by methods such as the Sachsseor Wulff processes, which utilize hydrocarbons or mixtures thereof asthe starting materials. The composition of a dilute acetylene streamobtained from the above processes depends, to a great extent, upon thehydrocarbon or hydrocarbons employed as the starting material. However,these streams generally contain various proportions of acetylene, carbonoxides, hydrogen, nitrogen and unsaturated and/or saturatedhydrocarbons. The acetylene can be recovered from this stream bycontacting it with a slurry of water and activated charcoal maintained,for example, at room temperature and at a pressure of about 50 p. s. i.g. Substantially all of the acetylene in the gas stream is adsorbed bythe charcoal and can be recovered therefrom preferably by a combinationof heating the acetylene-rich adsorbent at a somewhat lower pressure, e.g., 15 p. s. i. g.

In one preferred embodiment of the invention, water slurries containing5 percent by Weight or more of activated charcoal are preferable. Theadsorptive powers of The slurry is like a liquid and 4 removal ofhydrogen sulfide.

The slurry of water and charcoal can also be used as a combinationadsorption-absorption system. With this system, it is possible to obtaina fractionation between two dissolved gases, one more soluble in waterand one more soluble or capable of being adsorbed on the charcoal. Afterthe absorption and adsorption, the water and the charcoal can beseparated from each other by suitable filtration means and regeneratedindividually, thus giving a separation between the two gases. Systemswhich can be separated by this means are mixtures containing hydrogencyanide and ammonia, hydrogen sulfide and carbon dioxide, andacrylonitrile and hydrogen cyanide.

Since certain changes may be made in the above process without departingfrom the scope of the invention herein involved, it is intended that allmatter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:

1. A process for the separation of gases which can be adsorbed oncharcoal from gases soluble in water from a gaseous mixture containingthe same which comprises contacting said gaseous mixture with a slurryof water and activated charcoal, maintained under pressure, separatingthe water and the charcoal, and recovering the gases adsorbed on saidcharcoal and the gases absorbed in said water.

2. A process for the separation of hydrogen cyanide and ammonia fromeach other which comprises contacting a gaseous mixture containinghydrogen cyanide and ammonia with a slurry of water and activatedcharcoal, separating the water and the charcoal, and recovering thehydrogen cyanide adsorbed on said charcoal and the ammonia absorbed insaid water.

3. A process for the separation of hydrogen sulfide and carbon dioxidefrom each other which comprises contacting a gaseous mixture containinghydrogen sulfide and carbon dioxide with a slurry of water and activatedcharcoal, separating the water and the charcoal, and recovering thehydrogen sulfide adsorbed on said charcoal and the carbon dioxideabsorbed in said water.

4. A process for the separation of acrylonitrile and hydrogen cyanidefrom each other which comprises contacting a gaseous mixture containingacrylonitrile and hydrogen cyanide with a slurry of water and activatedcharcoal, separating the water and the charcoal, and recovering theacrylonitrile adsorbed on said charcoal and the hydrogen cyanideabsorbed in said water.

References Cited in the file of this patent

1. A PROCESS FOR THE SEPARATION OF GASES WHICH CAN BEADSORBED ONCHARCOAL FROM GASES SOLUBLE IN WATER FROM A GASEOUS MIXTURE CONTAININGTHE SAME WHICH COMPRISES CONTACTING SAID GASEOUS MIXTURE WITH A SLURRYOF WATER AND ACTIVATED CHARCOAL, MAINTAINED UNDER PRESSURE, SEPARATINGTHE WATER AND THE CHARCOAL, AND RECOVERING THE GASES ADSORBED ON SAIDCHARCOAL AND THE GASES ADSORBED IN SAID WATER.